Engineering plastics are those that can be molded into shapes such as gear wheels and can retain these shapes against impact and strain. In order to meet these requirements, the plastic should not crystallize but remain amorphous. The development of crystallinity in the plastic reduces its resistance to impact. As a rule of thumb, clear plastics are amorphous, while opaque plastics contain a high degree of crystallinity. In addition to remaining amorphous, the plastics must retain their tensile strength to resist breaking under strain. Many plastics which when dry display high tensile strengths lose tensile strength by absorbing small amounts of moisture from the atmosphere. Ideally, engineering plastics neither become crystalline nor sensitive to moisture.